The present invention relates generally to ink-jet printing, and, more particularly, to improving the properties of an ink-receiving layer applied to a non-absorbent substrate.
Ink-jet receiving layers need to absorb the ink vehicle delivered during the printing process. When the ink-receiving layer is applied to non-absorbent substrate, the substrate provides no absorption capacity and as a result, the ink-receiving layer must be the sole absorbing material. To increase the absorbing capacity of the coating, an absorbent precoat has been described in the prior art that serves to increase the capacity of the coating, much as a substrate functions in paper-based ink-jet media.
A topcoat is applied to control surface properties such as gloss, tackiness, surface energy, and durability, as well as to function in concert with the adsorbent precoat. In addition, the topcoat must be free of defects that would contribute to perceived irregularities or non-uniformities in the coating.
U.S. Pat. No. 5,275,867 describes a two-layer coating and a coating process where a topcoat is laminated on the precoat. U.S. Pat. No. 5,605,750 describes a three-layer coating and a coating process where the topcoats are applied to the precoat by coating both fluids before drying in a multi-slot hopper or a slide hopper. U.S. Pat. No. 5,576,088 describes a two layer coating and a coating process where a topcoat is cast coated on a precoat. All these examples describe a process that involves specialized equipment and coatings engineered to be compatible with the processes. In addition, production efficiencies may be lower.
An on-going problem in the application of a topcoat with basic coating equipment such as mayer rod and slot die coaters is the formation of bubbles in the topcoat when it is coated on a porous basecoat that has been applied to a non-porous substrate. These bubbles are formed when the air voids in the pores of the precoat are filled with fluid from the topcoat application process which results in the air being forced to surface of the precoat where they coalesce into bubbles in a still fluid topcoat. These bubbles can then form defects in the topcoat as that coating is dried. Another challenge when developing coating fluids and chemistries is avoiding problems associated with incompatible chemistries that result in solution gelling or phase separation in the dried coatings.
Thus, what is needed is a process that avoids the problems of the prior art and provides a uniform and defect-free topcoat layer, and thus allows the incorporation of incompatible chemistries into the coating.
The present inventors describe herein a process that allows the production of multi-layer coatings in which one or more topcoats can be applied to a porous basecoat to produce a uniform and defect-free coating layer. Specifically, a process is provided in which a liquid is applied to the basecoat prior to topcoating such that the air in the basecoat is removed prior to topcoating. This process can occur in-line with a simple apparatus described herein. An added benefit of this method is that it also allows the possibility of adding functionality or performing chemistry to the coatings after the basecoat is dried and before the topcoat is applied in a single process. For example, the wetting liquid may contain, but is not limited to, surfactants, pH modifiers, polymers, crosslinkers, pigments, and/or dye stabilizers.
Advantages of the invention over what has been done before include the use of rewetting process that allows a topcoat to be applied to a porous basecoat that is coated on a non-porous, or non-permeable, substrate such that bubbles are not formed in the topcoat. This allows the production of defect-free coatings. In addition, there is added flexibility of incorporating functionality or chemistry in the re-wetting process. Finally, the process of the present invention is simple to implement and is compatible with many general coating methods, such as slot-die coating, rod coating, blade coating, gravure coating, knife-over-roll coating, or the like.
An additional benefit of the above technique is that chemicals may be added to a coating which would otherwise be incompatible in the coating solution itself or the dried coating.
A still further benefit of the above technique is that two coating layers may be applied where incompatibilities may present difficulties in a wet-on-wet coating application technique.